1. Field of the Invention
The present invention relates to migration imaging apparatus and more specifically to improved structure for forming an imaging layer of photoelectrophoretic dispersion on electrode structure of such apparatus.
2. Description of the Prior Art
Briefly, conventional photoelectrophoretic migration imaging involving presenting an imaging layer (including a dispersion of photoconductive toner particles in a carrier or binder) in an electrical field between opposing electrode surfaces and imagewise exposing the layer. The opposing electrodes are often referred to as blocking and injecting electrodes (respectively having an insulating and conductive interface with the imaging layer); and, upon imagewise exposure, the toner particles migrate in an imagewise manner to form negative and positive images on the respective electrodes. There are a number of variations of this imaging technique. For example, U.S. Pat. No. 3,976,485 discloses a photoimmobilized migration imaging approach (wherein a negative image forms on the blocking electrode instead of the injecting electrode), and U.S. Pat. No. 4,168,118 discloses a field address photoelectophoretic technique (wherein discrete pixels of the image lyer are addressed by discrete electrical fields in the presence of flood illumination).
In the above and other similar imaging approaches using such an imaging dispersion, it is important to provide a dispersion layer of uniform thickness in the imaging zone (i.e. in the presence of the imaging field and illumination). A usual way of effecting this requirement is to provide an applicator that forms such a layer of the dispersion on one of the electrodes as the electrode moves from an upstream position to the imaging zone.
Although no commercial apparatus has yet evolved from this technology, a number of different, structures for continuous operation have been described, e.g. in patent literature. In such apparatus descriptions, several different dispersion application approaches are proposed. The most frequently proposed approach appears to be using an extrusion device which forms a bead of dispersion on the electrode, in cooperation with a doctor blade, which skives the extruded layer to the desired thickness. Another commonly disclosed approach is to use a donor roller that is partially immersed in a supply of dispersion, and which on rotation carries dispersion from the supply to an electrode. Yet another approach is to move the electrode directly through such a dispersion supply container and then smooth the layer with a downstream doctor blade on roller.
While all of the above applicator techniques appear technically feasible, I have found that, for certain commercial applications, those techniques present potential difficulties. More particularly, I have noted that prior art modes of applying dispersion all impart perturbations that cause the velocity of one or both of the apparatus electrodes to vary at a relatively high frequency. Such variation of the electrode velocity, commonly called "flutter", is particularly detrimental when the electrodes are addressed on a line-by-line basis, e.g. with a scanning laser modulated by an electronic image signal. In that kind of imaging mode electrode velocity "flutter" creates a highly objectionable image artifact termed "banding", which is a high frequency density variation within the image. This objectionable density variation occurs because the above-noted electrode velocity variations cause non-uniformity in the line-to-line spacing of scanned picture elements. Thus in migration imaging applications of the kind described above and analogous applications, it is particularly desirable that imaging dispersion be applied to the electrode in a manner avoiding perturbation of the electrode movement.